In message <022101cc4970$b24a4b80$16dee280$@com>, "Jose Camara" writes:
>After >one year of NTP queries, assume you have a 100ms jitter on the network time, >you could at most tweak your oscillator, based on past performance, to 6E-9. It is a lot more complicated than that, we need to talk allan-deviation here, not scalar numbers. The main problem here is that the 'default' NTPd software is not really written for something like this, and has attributes which makes it truly sucky for the task. If you want to do this, you want to write your own software and you want to give it an entirely different modus operandi. As with all oscillator discplining, what you are looking for is the so called "allan intercept" where the two sources allan deviation cross. With a NTP reference, its location varies depending on stochastic network properties, which depends what's between the server and you. If you control the network topology (as in: Can make sure there is no other traffic), you're fine, normal PLL style stuff works. If you don't control the network topology, the RTT between you and the server becomes a BIG problem, because the fundamental NTP assumption that it is symmetric is almost always wrong. You can average over long tau's, but then your ISP upgrades their routed and a systematic change in RTT screws your integrator over for several weeks. Alternatively, if your LO is stable enough (=Rb/Cs), you can operate on first derivative of the RTT, which turns the routed upgrade into a single spiky sample, but the cost is an overall higher noise in your error signal. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.